Hydrogen offers several advantages over petroleum based fuels in terms of reduced emissions and improved fuel efficiency. For example, when hydrogen is used in fuel cells to produce electricity for powering electric motors, the byproduct is water. When hydrogen is burned in an internal combustion engine such as a turbine engine or a piston engine, exhaust gas emissions are relatively low as compared to the emissions resulting from the combustion of petroleum based fuels. Another advantage of hydrogen as a fuel is the generally higher energy-per-mass density as compared to petroleum based fuels such as jet fuel. For example, hydrogen contains approximately three times the energy-per-unit mass of petroleum based fuels.
Hydrogen may be efficiently stored in liquid form in order to minimize the required storage volume. Although storing hydrogen in liquid form requires maintaining the temperature below approximately −420° F., the low pressure at which liquid hydrogen may be stored minimizes the overall weight of the vehicle as compared to the large number of tanks that would be required to store hydrogen in gaseous form.
The above-noted advantages associated with hydrogen may be applied to certain vehicles. For example, high-altitude, long-endurance (HALE) aircraft may benefit from a hydrogen-based propulsion system. HALE aircraft may be designed to operate at altitudes of up to 65,000 feet and may stay aloft for up to fourteen days or longer. However, a variety of other vehicles and systems may benefit from hydrogen as an alternative to petroleum based fuels.
In order to use hydrogen in a fuel cell or in an internal combustion engine, the hydrogen must be in a gaseous state. In addition, it is necessary to increase the pressure of the gaseous hydrogen to suit the operating requirements of the fuel cell or internal combustion engine. Prior art methods for converting liquid hydrogen to gaseous hydrogen at a suitable temperature and pressure include the use of heat exchangers and mechanical pumps. Heat exchangers may be used to vaporize the liquid hydrogen into gaseous hydrogen for use as a fuel. Unfortunately, heat exchangers are typically bulky.
In long endurance applications such as HALE aircraft, mechanical pumps must be capable of operating continuously for extended periods of time. The extremely low temperature of liquid hydrogen and the low viscosity of hydrogen limits the efficiency and reliability of mechanical pumps. More specifically, because of the extremely low temperatures, portions of the mechanical pump that are exposed to the liquid hydrogen may undergo significant thermal contraction. In order to accommodate differences in thermal contraction between various portions of the mechanical pump, the mating components of the pump must be designed and manufactured with relatively large tolerances. However, large tolerances may reduce pump efficiency.
In addition, mechanical pumps typically include rotating components which require lubrication to minimize friction and prevent wear. Unfortunately, the relatively low viscosity of liquid hydrogen minimizes the ability of the hydrogen to act as a lubricant. Furthermore, the low temperature of liquid hydrogen minimizes the available number of compatible lubricants (e.g., Teflon) that may be used in the pump.
As can be seen, there exists a need in the art for a system and method for converting liquid hydrogen into gaseous hydrogen at a suitable operating temperature and pressure. In this regard, there exists a need in the art for a system and method for converting liquid hydrogen into gaseous hydrogen which requires a minimum number of moving parts and wherein gaseous hydrogen may be continuously produced in a reliable and efficient manner.